I have found out what those new round devices on top of the Tube turnstiles are: they are smartcard scanners. One day, you'll have a card containing a chip, which you wil be able to recharge at a machine in the station, and which you will simply wave over the scanner to enter. The correct fare will be deducted from your balance.

And when will this card be introduced? Neither of the LU officials I asked was able to answer that. "They've been talking about it for 18 months," said one. "It'll probably take years," said another.